The present invention relates to data transmission over telecommunications networks, and in particular to the transmission of digital data over speech channels of such networks.
Using speech channels for the transmission of data over networks can have advantages under some circumstances because they are generally given a high priority and are subject to relatively small delays. This is necessary to ensure that normal speech can be transmitted without unacceptable loss of clarity, whereas data channels are generally susceptible to greater delay. However transmitting non-speech data over speech channels can be difficult, especially where low bit rate speech coding systems are used, such as in GSM. These systems have been developed recently, and have data rates of below 50 kb/s and often below 30 kb/s. The bit rate that can be used for speed coding can be even lower, and may be less than 20 kb/s. For example, a typical GSM voice channel has a 22.8 kb/s rate, but only 13 kb/s of the gross bit rate is used for speech coding.
One problem that systems such as the GSM system do not fully address is security. The GSM system ensures subscriber identity confidentiality and provides subscriber authentication, as well as confidentiality of user traffic and signaling. The ciphering algorithms used in GSM have proved to be effective in ensuring traffic confidentiality. However, the traffic confidentiality is only maintained across the radio access channel. Voice traffic is transmitted across the core circuit-switched networks ‘in-clear’ in the form of PCM or ADPCM speech, which opens up the possibility of unauthorized access, to GSM to GSM, or GSM to PSTN conversations. For end-to-end security the speech signal must be encrypted. This means that it no longer resembles speech, and cannot therefore be sent over the voice channel in the same way as un-encrypted speech. Instead it needs to be considered in the same way as general digital data. A disadvantage of the GSM speech channel is that security is controlled by the network operator, not the end user. Control by the end user may be preferable in some applications.
Although the GSM data channel can be used for encrypted speech transmission, this approach suffers from a number of disadvantages, in particular delays as mentioned above. The GSM data channel typically requires 28 to 31 seconds to establish a connection, of which approximately 18 seconds are taken up by the GSM modem handshaking time. In addition, the round trip time of the GSM data channel is between 1 and 2 seconds for the 95th percentile.